1. Field of the Invention
This invention relates generally to the fields of microbiology and biochemistry. Specifically, the present invention is related to a recombinant host cell, in particular a yeast cell, comprising a dihydroxy-acid dehydratase polypeptide. The invention is also related to a recombinant host cell having increased specific activity of the dihydroxy-acid dehydratase polypeptide as a result of increased expression of the polypeptide, modulation of the Fe—S cluster biosynthesis activity of the cell, or a combination thereof. The present invention also includes methods of using the host cells, as well as methods for identifying polypeptides that increase the flux in an Fe—S cluster biosynthesis pathway in a host cell.
2. Background of the Invention
Iron-sulfur (Fe—S) clusters serve as cofactors or prosthetic groups essential for the normal function of the class of proteins that contain them. In the class of Fe—S cluster containing proteins, the Fe—S clusters have been found to play several roles. When proteins of this class are first synthesized by the cell, they lack the Fe—S clusters required for their proper function and are referred to as apoproteins. Fe—S clusters are made in a series of reactions by proteins involved in Fe—S cluster biosynthesis and are transferred to the apo-proteins to form the functional Fe—S cluster containing holoproteins.
One such protein that requires Fe—S clusters for proper function is dihydroxy-acid dehydratase (DHAD) (E.C. 4.2.1.9). DHAD catalyzes the conversion of 2,3-dihydroxyisovalerate to α-ketoisovalerate, and of 2,3-dihydroxymethylvalerate to α-ketomethylvalerate. The DHAD enzyme is part of naturally occurring biosynthetic pathways producing the branched chain amino acids, (i.e., valine, isoleucine, leucine), and pantothenic acid (vitamin B5). DHAD catalyzed conversion of 2,3-dihydroxyisovalerate to α-ketoisovalerate is also a common step in the multiple isobutanol biosynthetic pathways that are disclosed in U.S. Patent Appl. Pub. No. US 20070092957 A1, incorporated by reference herein. Disclosed therein is, e.g., the engineering of recombinant microorganisms for the production of isobutanol.
High levels of DHAD activity are desired for increased production of products from biosynthetic pathways that include this enzyme activity, including, e.g., enhanced microbial production of branched chain amino acids, pantothenic acid, and isobutanol. Isobutanol, in particular, is useful as a fuel additive, and its ready availability may reduce the demand for petrochemical fuels. However, since all known DHAD enzymes require a Fe—S cluster for their function, they must be expressed in a host having the genetic machinery to provide the Fe—S clusters required by these proteins. In yeast, mitochondria play an essential role in Fe—S cluster biosynthesis. If the DHAD is to be functionally expressed in yeast cytosol, a system to transport the requisite Fe—S precursor or signal from mitochondria and assemble the Fe—S cluster on the cytosolic apoprotein is required. Prior to the work of the present inventors, it was previously unknown whether yeast could provide Fe—S clusters for any DHAD located in the cytoplasm (since native yeast DHAD is located in the mitochondria) and more importantly when the DHAD is expressed at high levels in the cytoplasm
Under certain conditions the rate of synthesis of Fe—S cluster requiring apo-proteins may exceed the cell's ability to synthesize and assemble Fe—S clusters for them. Cluster-less apo-proteins that accumulate under these conditions cannot carry out their normal function. Such conditions can include 1) the expression of a heterologous Fe—S cluster requiring protein especially in high amounts, 2) the expression of a native Fe—S cluster biosynthesis protein at higher levels than normal, or 3) a state where the host cell's ability to synthesize Fe—S clusters is debilitated.